1. Field of the Invention
The present invention relates to a developing method of a resist pattern using immersion lithography, solution used for the developing method, and to an electronic device formed using the developing method.
2. Description of the Background Art
In immersion exposure (immersion lithography), a water film (meniscus) is formed utilizing surface tension of water in a small space between a lens of an exposure apparatus and a wafer as an object of processing, to attain higher refraction index between the lens and a surface to be irradiated (surface of the uppermost film on the wafer). Effective numerical aperture (NA) of the lens can be increased to be higher than in common dry exposure, to approximately 1.44, that is, the refraction index of water. Therefore, immersion lithography has been put into industrial practice as a technique that increases resolution limit of lithography and enables miniaturization of patterns formed.
FIG. 1 is a schematic illustration of immersion lithography. As shown in FIG. 1, in immersion lithography, water 5 is provided as a meniscus 5a in a small space between a lens 3 and a photoresist layer 2 formed on the uppermost surface of a wafer 1 placed on a stage 6, and wafer 1 is scanned (as represented by an arrow A in FIG. 1) while it is irradiated with light through meniscus 5a, to realize scanning exposure.
As described above, in immersion lithography, there is water 5 forming the water film between lens 3 and photoresist layer 2 as the surface to be irradiated (the surface of uppermost film on wafer 1) and, therefore, when a common chemically-amplified resist for dry exposure is used, it is possible that low molecular compound such as photo-acid-generating agent or base contained in the chemically-amplified resist dissolves to water 5, possibly causing contamination of exposure apparatus including lens 3. If the resist has low repellency, micro water drops possibly remain on photoresist layer 2 after meniscus 5a has been moved when the water film (meniscus 5a) is moved on wafer 1, so that the resist is kept locally in contact with water for a long time. This may lead to pattern defects.
In order to prevent low molecular compound such as photo-acid-generating agent or base in chemically-amplified resist from being dissolved to water 5 and to enable high-speed and smooth movement of meniscus without leaving droplets, a technique has been adopted, which makes it difficult for the low molecular compound to dissolve from the surface of photoresist layer 2 and provides repellency to the surface.
A specific example of such technique involves formation of an upper layer protective film (top-coat) that dissolves to a developer on the resist, to prevent direct contact between the water and the resist. Further, a top-coatless resist has been developed and commercially available, in which a small amount of polymers (mainly, fluorine-containing polymer) having low critical surface energy, such as water repellent agent, is mixed in the chemically-amplified resist, to have the water repellent agent concentrated (unevenly distributed) only to the resist surface utilizing surface segregation effect of water repellent agent when coating film is formed, so that the two layers of resist and top-coat are spontaneously formed as a single coating film.
The method of providing a top-coat is generally referred to as top-coat process, which proceeds through the process flow shown in FIG. 2(a). According to this method, a resist layer is formed as a two-layered film by two steps of application, in which a common photoresist film (ST2-a3) is formed and a resist upper layer protection film (top-coat) is applied thereon (ST2-a4). It is often the case that the top-coat is formed of fluorine-containing polymer with alkali-soluble part, to have a highly repellent film that is soluble to alkali developer and automatically peeled at the time of development (see, for example, Japanese Patent Laying-Open No. 2007-148167). The top-coat process, however, is disadvantageously redundant as it requires two steps of application (ST2-a3 and ST2-a4) for forming the resist layer and it leads to higher cost, as two different chemicals are used for forming the resist and the top-coat. Further, there is a trade-off between the resist containing developer-soluble base and having high repellency. Therefore, repellency is not always sufficient for exposure to attain high throughput by scanning at higher speed.
On the other hand, there is a method using a top-coatless resist, which proceeds through the process shown in FIG. 2(b) and includes the process as illustrated in FIG. 7. According to this method, a small amount of polymers (fluorine-containing polymer or the like) having low critical surface energy, such as water repellent agent mentioned above, is mixed with resist liquid as described above, and the resist layer is formed thereby (see, for example, Japanese Patent Laying-Open No. 2008-102276). The water repellent agent is concentrated (unevenly distributed) only to the resist surface because of the surface segregation effect of water repellent agent, whereby a surface that corresponds to the top-coat can spontaneously be formed by a single step of forming a coating (ST2-b2). Such top-coatless resists have been developed and come to be commercially available. The surface segregation phenomenon described above is exhibited based on a principle of thermodynamics that minimizes the entire free energy (sum of mixture free energy, surface free energy, interfacial free energy and the like) of the single coating system (resist layer). Therefore, with such a material, process redundancy of forming the photoresist film and the top-coat through different steps of application (ST2-a3 and ST2-a4) in the top-coat process described above can be avoided and, as a result, increase in cost of apparatuses including application cups for separate processes and bake plates as well as increase in material cost can be prevented.
Here, there are mainly three types of water repellent agents: (1) material that is soluble to developer, similar to the top-coat material; (2) water repellent agent not at all soluble to alkali; and (3) water repellent agent having property similar to chemically-amplified resist, which is de-protected by catalytic function of acid generated during exposure, and exposed portions of which become soluble to alkali developer at the subsequent post exposure bake (PEB).
The water repellent agent that changes to be soluble to alkali developer tends to have lower segregation characteristic when applied, and similar to the top-coat having the above-described problem, it often fails to attain sufficient repellency. As can be seen from schematic illustrations of various pattern defects of FIGS. 8(a) to 8(c), when a water repellent agent not having alkali solubility at all is used, the water repellent agent may be left as insoluble matter at the time of development, resulting in residue 10b (FIG. 8(a)), inducing a micro-bridge 10c (FIG. 8(b)) even when there is only a few residue, or leaving granular residue 10d (Blob) (FIG. 8(c)) since excessively high repellency hinders cleaning during rinsing with pure water after immersion in alkali liquid in the developing process.
When a water repellent agent that is de-protected by acid and only the exposed portion of which changes to be soluble to alkali developer in post exposure bake (PEB) is used, defect generation is still possible at the unexposed portion, as in the water repellent agent not at all having alkali solubility described above. Specifically, in forming an electronic device, background portions are unexposed at the hole forming step using a positive resist or at the trench forming step for Cu wiring using a dark field mask and, therefore, there is a high risk of defect generation.